The present invention relates to a scanner and particularly relates to a scanner which makes fluctuation movement by a micro-angle.
An example of a galvanometric scanner according to the background art has been disclosed in JP-A-2003-307700. As typically shown in FIG. 4, in the scanner disclosed in JP-A-2003-307700, a coil and permanent magnets are disposed circumferentially only in the fluctuation range of the galvanometric scanner. On this occasion, the coil is disposed on the rotor side and the permanent magnets are disposed on the stator side. The outer diameters of the coil and the permanent magnets are substantially constant.
On the other hand, an example of a DC motor provided with permanent magnets disposed around a rotation shaft has been described in JP-A-2004-88855. According to JP-A-2004-88855, permanent magnets are disposed in a surface of a rotor opposite to a stator in order to suppress distortion of an induced voltage as well as to suppress characteristic change caused by demagnetization. Moreover, grooves with a shape inclusive of a linear shape are formed axially in surfaces of the permanent magnets opposite to the stator so as to be near the magnetic pole boundaries of the permanent magnets.
In the background-art galvanometric scanner, there is the possibility that uniform torque cannot be obtained because the torque constant varies according to the rotation angle. That is, if the outer diameter of the permanent magnets forming the rotor is set to be substantially constant, magnetic flux density substantially describes a parabola in which the magnetic flux density is maximized in a circumferential center position of each permanent magnet and reduced toward opposite circumferential end portions of the permanent magnet. As a result, in the case of a four-pole DC motor, the torque constant is reduced by about 10% when the rotation angle changes by 15 degrees. This can apply to the DC motor described in JP-A-2004-88855.